In the realm of peripheral equipment for data processing systems, it is difficult to think of a device that is more reliable than a modern laser or ink-jet printer. In spite of their inherent reliability, such printers do require rather infrequent periodic maintenance. One of the items that is subject to degradation with use is the media feed, or pick, roller that is responsible for grabbing a single sheet of print media from a paper storage tray or bin and feeding the sheet into a series of transport rollers which direct the sheet along a path within the printer. Various types of print media, such as bond paper, cotton-fiber bond paper, recycled paper and plastic transparencies, may be used, depending on the type of printer. Typically the media feed roller is either cylindrical or a section of a cylinder (i.e., pie-shaped when a cross section is taken perpendicular to the cylindrical axis). As a general rule, media feed rollers have a resilient rubber outer coating, layer or sleeve that has a high coefficient of friction. There are three main reasons why feed rollers must be periodically replaced. Firstly, with use, the outer surface of the roller gradually wears, thereby reducing its radius. As the radius decreases, the area of the roller which contacts the upper most sheet of a stack of print media sheets under an applied pressure during a media feed operation also decreases. As the area of contact decreases, so does the ability to reliably pick up individual print media sheets. Secondly, normal aging of the rubber compound by, for example, exposure to ozone in the atmosphere, can reduce the coefficient of friction. Thirdly, the media feed roller typically becomes contaminated with abraded media particles. Paper, the most common print medium, contains a potpourri of ingredients, which may include cellulose fibers; rosin (added as size to decrease the rate at which cellulose fibers absorb water); fillers such as clay, titanium dioxide, talc, and calcium carbonate (found chiefly in recycled paper from Europe); and various dyes. As paper is abraded by the feed roller, a some of the resulting cellulose and filler particles (e.g., clay, titanium dioxide) become embedded in the surface of the roller. Other abraded particles (most notably the abietic acid molecules from the rosin and the various organic molecules from which the dyes are formed) become dissolved within the rubber itself. Once dissolved, they freely migrate within the rubber, thereby changing the chemical and mechanical characteristics of the rubber. Contamination of the rubber on the media feed roller invariably reduces the coefficient of friction, making paper feed failures increasingly likely. Feed failures may be take several forms which may include: a complete failure to feed the media sheet, a late feed of the sheet, or a jam related to differential feed rates from one side of the sheet to the other. Collectively, these media feed failures will be referred to as misfeeds. Though the roller may be cleaned with an organic solvent such as alcohol, its coefficient of friction is unlikely to be restored to original specifications. Of course, contaminants dissolved within the rubber cannot be removed by a simple cleaning. At some point, the media feed roller must be replaced in order to minimize media misfeeds and restore printer performance.
The misfeed rate caused by media feed roller malfunction follows an interesting pattern. For a high-end printer with a new media feed roller, the feed failure rate will be around 1 misfeed per 10,000 pages, with replacement of the roller being made when the failure rate is greater than about 1 misfeed per 500 pages. Using print media of low abrasiveness and low pollutant content, this excessive jam rate will begin to occur when the roller has fed about 350,000. For a new low-end printer, a somewhat higher feed roller related feed failure rate of about 1 misfeed per 3,000 pages is considered acceptable, and replacement is usually made when the failure rate is greater than about 1 misfeed per 100 pages. With ideal use, the unacceptably-high misfeed rate will be reached when the roller has fed about 100,000 pages.
What is needed is a method for alerting the user or a network administrator of the need to replace a media feed roller. In most instances, it is not enough to simply alert the user after a total number of pages have been printed which correspond to the expected life of the roller under ideal service conditions. Due to greatly varied printing conditions, the roller may require replacement sooner.